The present invention relates to a fuel control governor and particularly to such a governor for achieving rapid response and low leakage rate while also providing a device of minimum size and light weight having a small number of moving parts.
An aircraft turbine engine suitably employs a mechanical fuel control governor for protecting against overspeed conditions. The governor in typical form comprises a rotating assembly having flyweights which are urged outwardly by centrifugal force, the weights operating a valve for reducing fuel pressure as engine speed becomes excessive. The metering valve need not rotate with the assembly; however, relative rotation between a valve spindle and its surrounding sleeve is advantageous in eliminating the necessity of overcoming initial axial governor actuation friction, and for clearing small debris in the fuel as might otherwise interfere with valve operation. Rotation of a metering valve component is important if the governor is to respond smoothly, rapidly and reliably.
Maximizing rotational speed of the flyweight assembly is desirable in improving sensitivity to small speed changes. Centrifugal force increases in proportion to the square of the assembly speed whereby a difference in centrifugal force is more noticeable for speed changes in a high range than if the assembly were operating at a lower speed.
However, simultaneous rotation of a valve component at near input shaft speed can be unacceptable from the standpoint of minimizing valve leakage. Minimum valve leakage is desirable for improving governing sensitivity at design speed. Leakage can be lessened through reduction of clearances, e.g. between a rotating metering spindle and the surrounding sleeve. Although a lower limit in valve clearance is determined by the size of debris in the fuel, the latter issue can be addressed by incorporating an appropriate system fuel filter, whereby spindle speed then becomes the controlling factor. Since the spindle operates in fluid film bearings, reduction of spindle clearance increases the shear gradient across the fluid film while also reducing fuel flow along the bearing axis. Both of these factors increase heat generation and the likelihood of failure if the spindle is rotated too rapidly. Consequently, from the standpoint of low valve leakage, a lower spindle speed is more attractive.
Heretofore, the attainment of high flyweight assembly speed together with a low valve rotational speed consonant with minimizing leakage has been achieved by providing a geartrain between the flyweight assembly or drive and the metering spindle for relatively reducing spindle speed. This solution, while satisfying desired design criteria, sacrifices weight and space aspects of the governor.